Explain this to me please. Talk.origin article on Micro and Macro

I was reading this article in Talk.origins:
Macroevolution: Its definition, Philosophy and History)I'm confused about something. This particular paragraph:
"There is no difference between micro- and macroevolution except that genes between species usually diverge, while genes within species usually combine. The same processes that cause within-species evolution are responsible for above-species evolution, except that the processes that cause speciation include things that cannot happen to lesser groups, such as the evolution of different sexual apparatus (because, by definition, once organisms cannot interbreed, they are different species)."
Question:
How can there be "no" difference if ,geneticaly speaking, one "Diverges" while the other "Combines"? Are these not complete "Opposite" processes? And if they are both using the same process how is it that the "Same" process causes results that are completely "Opposite" from each other? This is giving me dame bramage , can anyone explain what this means?

I like this question a lot! Thank you. I haven't had a chance to look at it exactly that way before. I'm NOT an expert so take what I write with a tablespoon of NaCl, k? Here goes an explanation made up on the fly.Let's first clarify what I think we are talking about as far as mechanism. We have some overall population genonme with different individuals haveing some parts of it. During reproduction there are recombinations of the indivuduals genomes to produce novel ones. In addtion, there will be mutations of one form or another. Some individuals have genetic traits which enhance their reproductive success, some don't.Now within a species what happens. The different novel genetics may be mixed with and passed on to pretty much any individual within the population. They are all subject (if the population isn't too big) to similar selective pressures. This tends to push the indiviuals within the species population to not get too far from each other. Any harmful mutations will be selected out at some point (probably). Any neutral mutations have a chance to spread through the population (or fade away) and, perhaps be available if circumstance make them beneficial. Any really beneficial mutations (or recombinations) will tend to be fixed in the population and all individuals will carry them. This can happen only because the population is ONE species and ALL potentially interbreeding.The same mechanisms operating on populations that are not the same species produce novel genomes too. However, now the two populations are not going to mix their genomes. Thus any mutation that arises in an individual in one species has only a (possibly) small chance of also happening in the other separate species. If the chance of a duplicated mutation really are small and there is no way for the two distinct breeding populations (that is one way a species is defined) can not share the mutations then, over time, the two populations will, obviously diverge. One population will pick up one set of mutations and the other will pick up a different (mostly or completely ) set.[This message has been edited by NosyNed, 12-22-2003]

They may be or they may not be depending on how orthogonality and ortohogenesis relate to the relative frequency being asserted on/as a/the whole. Parallelism makes this indeed difficult to inspect without further information on the author's declination. Also the grammer may be fairly criticised as overdetermined comparing the lexos on some as to be agreed on standard of LOGOS.

You could use language as an analogy, this might make things a little clearer (as long as you don't take the analogy too far). As long as a population is in contact with each other they keep the same common language. However, the language may change over time but people will still understand each other at all times. An example is Old English and current English. If a small portion of the population is isolated from the larger population they might develop a different language so that the two populations can no longer communicate without translation. An example of this could be the Romance Languages that came from Latin (French, Italian, and Spanish). In both cases the common language changed but because of isolation divergence between the languages occured. Also, the mechanisms for language change overtime is the same in both cases.In biological systems the same thing can be seen. Genetic drift, Old English to current English in the analogy, occurs within the species but overall, gradual changes in the population are uniform. However, if a portion of a species is isolated, the same genetic drift will cause divergence in genetic makeup since the same genetic drift will not occur in both populations (Romance languages).Just in case:
Genetic drift: Changes in the genetic makeup in a population due to mutation and natural selection within a species. Usually used to describe neutral mutations, but could result in morphological changes over time. Not as dramatic as punctuated equilibrium.

Just a quibble, but as I understood it, natural selection isn’t involved in drift. If there’s selection, then it’s adaptation rather than drift. Your definition there is of evolution in general.From here:

quote:

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genetic drift -- In a small gene pool, the alteration in allele frequencies mostly by accidents rather than by natural selection.

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See also http://www.talkorigins.org/faqs/genetic-drift.htmlCheers, DT[This message has been edited by Darwinsterrier, 12-23-2003]

Hi Milagro,John Wilkins, the author of that FAQ, is usually a very good, clear writer. This effort was not one of his best, and I agree that the way he wrote the conclusion is confusing. However, if you look back at the top of the FAQ where he defines the terms, you'll see that in the paragraph you quoted he's merely restating the lineage definitions of macro- and microevolution in terms of what happens in the genomes.Let me see if I can clarify/simplify a bit. Macroevolution (according to John), from a lineage point of view, is the term used to describe the branching of lineages (the "node", if you like cladistics jargon). It can be either divergence ("cladogenesis", where a lineage spawns a daughter lineage but may continue to persist), or replacement ("anagenesis", where a particular lineage is so modified gradually over time from its ancestral form that taxonomists categorize it as a distinct species). Obviously, macroevolution is most easily observed in the fossil record over deep (geologic) time, since at any lesser time scale it can be difficult to identify the branch point. In essence, macroevolution is a description of pattern in speciation.Microevolution, on the other hand, refers to changes in the frequency of certain traits within a given population (or more broadly, species) of organisms - not a lineage - literally on a generation to generation basis. It is readily observable in short (ecological) time frames. Of course, that's why even creationists can't deny it.Let's see if I can explain it from a genes-eye view, so you can see what JW was trying to say. As you know, variation is constantly arising from one generation to the next in all interbreeding populations through mutation or genetic recombination. As long as the local environment in relatively stable (all other things being equal), the genetic make-up of the population - and the frequency of individual genes - will vary around a mean for the population as a whole.* Sometimes called stabilizing selection, this leads to the persistence of certain traits ("phenotypes" = suites of traits) in the population. Under stabilizing selection, evolution is NOT occurring, because trait frequency remains more or less static over time. However, when environmental factors change, natural selection will start "pushing" gene frequencies away from the original mean (known as "directional selection"). If the environmental changes are relatively permanent, then the new trait frequency will be permanent. This is microevolution. A graphic example of both types of selection occurred in the Grants' finches on Daphne Major (Galapagos Archipelago). The frequency of large and small-beaked Geospiza spp. had been relatively stable around an identifiable mean beak size due to annual vegetation changes (stabilizing selection). During a several-year drought, large-beaked finches came to predominate the community (directional selection) when the difference in life and death could be literally measured in 1-2 mm difference in beak size. Following the drought - since it wasn't permanent - the frequency gradually returned to pre-drought levels. However, if the environmental changes had been permanent (assuming it didn't simply result in the extinction of the whole population), the large beaks would have been the preferential survivors - and microevolution would have occurred within the population.Macroevolution happens when some permanent reproductive barrier arises between two or more groups of phenotypes within a population. Once the barrier arises - through whatever means - the genetic makeup of the two separated groups continues to diverge from each other by the action of natural selection operating on the continual variation that arises in each sub-population. Ultimately, over time, the sub-populations become different enough to be more conveniently classed as distinct species (the branch point or node). Over sufficiently longer time scales, those two original subpopulations may be so different that they can be classed into different genera or even families as each continues to throw up newer and ever more different phenotypes to the tender mercies of natural selection.Macroevolution occurs when the differences between two populations - caused by microevolution operating at the population level - creates enough distinction that we can say speciation has occurred. As JW pointed out, there's no fundamental difference between the two, except that microevolution describes changes in the internal genetic composition of a population, whereas macroevolution is the description of the divergence in the genetics of two different populations that leads to taxonomic reclassification. He just said it badly... *I am purposely excluding genetic drift from this discussion. It is something of a special case operating primarily on small populations. Basically, it is a stochastic (statistical) process - a random walk where frequencies change more or less by chance, rather than through the action of natural selection operating on variation. Most of the proposed examples of speciation (macroevolution) via genetic drift have been inconclusive (and to me unpersuasive), although microevolutionary change by this process is fairly well documented.I hope this helps explain what Prof. Wilkins was on about.

Thanks for the replies.In the "29 Evidences for Macroevolution" article (http:///RefLib/EvidencesMacroevolution.html) They're not even using Microevolution as a type of evidence to help support Macroevolution.
This is what the author writes:
The Hypothesis to be Tested: Universal Common Descent by Gradual ModificationIn this essay, macroevolution alone is specifically considered and weighed against the scientific evidence; in general, separate "microevolutionary" theories are left unaddressed. Microevolutionary theories, which biologists use as mechanistic theories to explain macroevolution, include such concepts as natural selection, genetic drift, sexual selection, neutral evolution, theories of speciation, etc. Whether microevolutionary theories are sufficient to account for macroevolutionary adaptations is a question that is left open. However, the fundamentals of genetics, developmental biology, molecular biology, biochemistry, and geology are assumed to be correct in general - especially those that do not directly purport to explain adaptation.So if NosyNed, not picking on you is correct in his analogy ,why not use a similar analogy to help support Macroevolution in this article? This surprises me. I read the article thinking it will show how Microevolutionary changes eventually lead up to Macroevolutionary ones and they refuse to use it in this article.Nevertheless if you guys can explain this one I'd be interested in hearing why they wouldn't use Micro to help support Macro.Thanks

Hi Quetzal-Good reply. Not to steer too far off topic, but I wonder if you might be able to answer another question for me - do speciation events coincide well with the Ice Age periods?Thanks for any replies.

Good question! I've honestly never thought to try and correlate it that way - I've focused primarily on the Ice Age -> extinction -> subsequent ecological release of survivors. I'll have to look around and see if there's been anything published specifically on your question. I wouldn't be surprised if there was some evidence of rapid speciation after Ice Ages. That seems to be a fairly common pattern after most large scale extinction events (regional or global). Gimme a bit of time to check it out.

Did I use an analogy?The FAQ you refer to isn't supposed to be a boook. It is, to me, attempting (well or poorly) the differences in the terms micro and macro evolution. I don't think it's trying to "support" macro evolution in that FAQ. It's just giving some history and discussion around the terms and the meanings to biologists.------------------
Common sense isn't

quote:

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Genetic drift: Changes in the genetic makeup in a population due to mutation and natural selection within a species.

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A further quibble: not only is natural selection not part of drift, neither is mutation, at least as the term is usually used. Drift refers to random fluctuations in the frequencies of existing alleles.

sfs and DT,Thanks to both. I wrote that in a hurry late last night, thanks for the correction. I guess RM+NS would actually be a better term with relation to species divergence.

I got you mixed up with another response I was reading on a similar topic. My bad!

MO,I haven't found anything that shows uncontroversial speciation as a direct result of the ice age itself (i.e. attributable to the climate change). There is quite a large amount of information showing the ice age(s) and the interglacial periods had indirect (secondary and tertiary) effects on species diversity leading to local and regional changes in biota: for instance faunal and floral turnover, episodic recolonization of continental islands leading to competitive exclusion and/or character displacement, extinction followed by adaptive radiation, habitat restriction to refugia leading to genetic divergence or bottleneck, etc.Thinking about it, that kind of makes sense. I am hard pressed to see how any population of organisms beyond simple algae or bacteria could have survived on the ice itself long enough to adapt. We're talking a frigid, sterile wasteland similar to the interior of Antarctica. Any population caught in front of the expanding ice front would be forced to either habitat track or go extinct. It was relatively quick process in any terms. Sorry if this sort of begs your question. Maybe someone else might have a better answer.

Milagros, if you'll look at predictions 22, 23, and 24 in that FAQ, those are pretty much microevolution -> macroevolution "transitionals".